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How does the shadows slider work in Adobe Camera Raw/Lightroom?

Naively I would expect the shadows slider to affect a simple curves boost in the bottom left corner, but it's doing more than that. It seems to be doing some kind of tone mapping/pseudo-HDR thing. How does it work? Is it documented somewhere?

Possibly the highlights slider might work the same, not sure.

Related: What do all the settings do in Lightroom?

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  • What adjustments have you tried? What were the results? How did these results differ from your expectations? – user50888 Dec 7 '17 at 18:45
  • Can you post some images that illustrate the difficulties you are having with the shadows slider? – user50888 Dec 7 '17 at 18:46
  • I don't have any difficulties, I just want to understand in-depth how it all works. – Aram Hăvărneanu Dec 7 '17 at 18:49
  • Considering your comments on the answer below, it is unclear to me what you are asking. – user50888 Dec 8 '17 at 20:57
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It seems to be doing some kind of tone mapping/pseudo-HDR thing.

Here's what a linearly depicted properly exposed image looks like:

enter image description here

Here's a thumbnail preview of the same image with a typical light curve added:

enter image description here

Everything we do with raw images captured using an imaging sensor that is linear in its response curve to light is "some kind of tone mapping/pseudo-HDR thing." That's what an adjustment to a light curve is! We're altering the relative difference in brightness between areas where more or less light was recorded when the image was taken.

This was also the case in the film era, except that the film emulsions themselves were designed to be more or less contrasty and to shape the curves at both ends of the dynamic range flatter or steeper.

As Ansel Adams perfected in the development of his 'Zone System', altering the exposure time and compensating with a longer or shorter development time alters the shape of the response curve for the same film emulsion. But Adams didn't stop there. He dodged and burned when making his prints by blocking certain areas of the light shining through his negative in the enlarger. Areas of the photosensitive paper that got light from the enlarger for less time printed lighter and were said to be "dodged." Areas of the paper that got light from the enlarger for more time printed darker and were said to be "burned."

Moving the "Shadows" slider in ACR raises the brightness of the dimmest parts of the image without doing the same to the mid-tones and highlights. It does so in a similar manner to the way Adams (and everyone who followed him) would reduce the contrast of his images by altering his exposure and development times so that there was not as great a difference between the darkest parts and the rest of the image.

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  • Sorry Michael, but this is incorrect. A TRC adjustment is not tone mapping in the general sense. A TRC is a (single) global operator, a function f: Rⁿ → Rⁿ, where n is the dimensionality of the color space. Tone mapping in general encompasses both global and local operators. With local operators you have a whole set (not a single) of operators parametrized by spatial coordinate OPs = {fₓ: Rⁿ → Rⁿ | x ∈ ImageCoord}. When talking about HDR you exclusively talk about local operators. – Aram Hăvărneanu Dec 8 '17 at 11:49
  • Yes, selective dodge and burn is definitely an example of HDR done with film, however this doesn't explain exactly, precisely, how ACR is doing it. – Aram Hăvărneanu Dec 8 '17 at 11:49
  • Your last paragraph about "moving the shadows slider in ACR raises the brightness of the dimmest parts of the image without doing the same to the mid-tones and highlights." is not true. You are describing just basic curves adjustment, and the slider is doing more than that. This is the purpose of this question, to determine what exactly is it doing that's more than that. It's applying local operators (selective dodge and burn), not just a global operator. – Aram Hăvărneanu Dec 8 '17 at 11:49
  • As a side note, I am an engineer and a programmer very familiar with signal processing and image encoding. I am very familiar with color science in general and I am trying to gain a deeper understanding in the practical (software) aspects of color. I am programatically creating my own color profiles, etc. – Aram Hăvărneanu Dec 8 '17 at 11:49
  • And as a film aficionado (if anything, I'm more of a film photographer than a digital photographer) I am very aware of the tone response of film in general. I even made my own emulsions many years ago. – Aram Hăvărneanu Dec 8 '17 at 11:49

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